Below:

NEW YORK — In a “Rock Center with Brian Williams” web exclusive, U.S. officials tell NBC News’ Richard Engel and Robert Windrem that deadly attacks on Iranian nuclear scientists are being carried out by an Iranian dissident group that is financed, trained and armed by Israel’s secret service. The group, the People’s Mojahedin of Iran, has been designated as a terrorist group by the United States.

The attacks have killed five Iranian nuclear scientists since 2007 and may have destroyed a missile research and development site. Mostafa Ahamdi Roshan died in a blast in Tehran on January 2012, moments after two assailants on a motorcycle placed a small magnetic bomb on his vehicle. Roshan was a deputy director at the Natanz uranium enrichment facility and was reportedly involved in procurement for the nuclear program, which Iran insists is not a weapons program.

Massoud Ali-Mohammadi was killed by a bomb outside his Tehran home in January 2010, and an explosion in November of that year that took the life of Majid Shahriari and wounded Fereydoun Abbasi-Davani, who is now the head of Iran’s Atomic Energy Organization.

Experts say the sophistication of the attacks supports the Iranian claims that an experienced intelligence service is involved.

If used, must include mandatory credit: “Rock Center with Brian Williams.” For the full story: http://bit.ly/xV09Lz

David Corvo is the senior executive producer, Rome Hartman is the executive producer and Brian Williams is the anchor and managing editor of “Rock Center with Brian Williams” (Wednesdays at 9p/8c). Follow us at RockCenterNBC.com, facebook.com/rockcenterNBC and @rockcenterNBC.

In a highly-anticipated speech to Congress Tuesday, Israeli Prime Minister Benjamin Netanyahu argued that a potential nuclear deal being negotiated by major powers including the United States "paves Iran's path to the bomb."